A couple years back I bought a 80something year old house. It has a Trianco DBC 100 Oil combi boiler that I'm guessing to be about ~15 years old. The system also has an old hot water tank strapped to the ceiling. It's practically a space heater in a (usually) unheated basement. I wrapped the tank with some spare fiberglass insulation, and that helped cut back on the boiler cycling frequently. We've had problems a handfull of times where sediment (or corrosion) caused clogs between the tank and boiler. The original plan was to switch to natural gas sooner than later, but the recent low oil prices, and a spray foamed attic have reduced the urgency.

I'm under the impression that storage tanks on combis can be unnecessary. What are the chances this tank is a holdover from an older setup? Does having the storage tank help with hot water demand? It's a two-bath house with dishwasher and washing machine. We don't normally run all four at the same time...but it could happen if we had a house full of visitors around the holidays.

Without seeing pictures, it's hard to tell. Normally all of this equipment would have name plates on it.

In my opinion though, the efficiency of newer water heaters, including TANK water heaters, are so vastly superior to the old water heaters / boilers, that the cost of replacement can get paid off within the first year of having it installed. The newer ones, particularly in the past couple of years, have dramatically more insulation. They often require more maintenance since they usually don't have glass tanks anymore, but they are considerably smaller since everything is a contained unit.

Also, the added benefit is that they're taller than they are widers, so you can be more specific to where you want to put it. Relocating the water heater to a place that's more convenient to you (into a utility closet, or just out of the way like under the stairs in the basement) is a lot easier now.

With natural gas, you don't really need a tank, you can go with a tankless, but like you were curious about, if you have a morning with two guests showering at the same time, and run the dishwasher... you're going to run into problems without a tank.

Some of the newer water heaters have computer control on them to regulate the gas or electric nodes. This allows you to save even more power / gas as it learns your household habits. They also have "vacation mode" and other things.

One of the best decisions I had was to swap out an older water heater from the 90s with a Whirlpool water heater with computer control / learn feature, and I immediately saw a drop by about $30-40 per month on a 2,300 square foot home w/ 3 people in the household.

I would also echo Todd's comment as to supplying some photos of your present HVAC setup-------what you describe as a hot water tank heater strapped to the ceiling, could well be something else-----like an EXPANSION TANK, an item that is required for all hot water heating systems, like yours.

I would also strongly recommend you have a licensed hydronic-hot water- technician come into the boiler room-their first visit is almost always free of charge- to point out the function of each of the HWH components and give his/her opinion of what to do next, after identifying the various components of your HWH system---I think you will have a much clearer picture of your system and what is needed.

People normally don't believe me when I say the tank is strapped to the ceiling. If we get a new guy for the annual tune-up, the first response is usually, "What that..." I don't think I saw a plate on the tank before I wrapped it. Next time I drain the expansion tank I'll unwrap the water tank and see if there's anything. One of the more seasoned guys that comes in thinks it's an old concrete lined monstrosity.